1. Technical Field
This invention relates to a retarding grid and in particular a spherical retarding grid used for energy resolution in charged particle systems.
2. Prior Art
Retarding grids are commonly employed in a variety of different charged particle systems, such as electron energy instruments and spectroscopy applications typified by Auger, photoemission, and energy loss spectroscopy. Typical prior art systems employ a wire mesh retarding grid. Such are limited in size due to manufacturing instability considerations or dynamic constraints.
Reference is made to Wei et al, "Instrumental Effects of the Retarding Grids in a LEED Apparatus", The Review of Scientific Instruments, Vol. 40, No. 8, pp. 1075-1079, August 1969, and Weber et al, "Use of LEED Apparatus for the Detection and Identification of Surface Contaminants", J. App. Phy., Vol. 38, No. 11, pp. 4355-4358, October 1967, which discuss wire mesh grid systems in the context of low energy electron defraction (LEED). The use of a mesh electrode for a cathode ray tube is set forth in Kokai No. 55-62790, May 14, 1980, wherein a planar mesh deforms under exposure to a high pressure gas to assume a curved shape conforming to a predefined cavity portion.
Another example of flexible grids for purposes of achieving beam concentration by having the grid deflect as a function of the voltage difference between it and generated ion plasma is disclosed in U.S. Pat. No. 4,538,067. Set forth therein, degree of focusing is a function of bowing of the flexible grid for purposes of extracting and accelerating ions from an ion plasma with the resulting high energy ion beam focussed onto a small area. The technique of manufacturing curved ion thrust to grids is disclosed in U.S. Pat. No. 3,864,797. The technique employed therein involves a simultaneous manufacture of grid pairs, an acceleration grid and a screen grid by employing a forcing fluid to inflate an impervious elastic sheet which contacts the grid blanks which forces bowing. Such grids are used as a part of an ion thrust of accelerator systems defined in the '797 patent for use as a thruster for spacecraft.
In terms of electron spectroscopy, the use of spherical grids is disclosed in Palmberg, "Optimization of Auger Electron Spectroscopy in LEED Systems", App. Phy. Let., Vol. 13, No. 5, pp. 183-185, September 1968; Taylor, "Resolution and Sensitivity Considerations of an Auger Electron Spectrometer Based on Display LEED Optics", The Review of Scientific Instruments, Vol. 40, No. 6, pp. 792-804, June 1969; Samson et al, "Photoelectron Spectoscopy of the Rare Gases", The Physical Review, Vol. 173, pp. 80-85 (1968); and Huchital et al, "High-Sensitivity Electron Spectrometer", App. Phy. Let., Vol. 16, No. 9, pp. 348-351, May 1970. In more general terms, analysis of spherical-grid retarding analyzers which summarizes and comments on many of the just cited citations is found in Huchital et al, "Resolution and Sensitivity of the Spherical-Grid Retarding Potential Analyzer", J. Appl. Phys., Vol. 43, No. 5, pp. 2291-2302, May 1972. As illustrated in FIG. 6 of that reference and disclosed throughout the body of the text, spherical retarding grid analyzers have been employed for many types of charged particles spectroscopy applications. The energy resolution of such devices is given by ##EQU1## where: r is the radius of the charged particle source and R is the radius of the spherical analyzer. As set forth in Huchital et al, there is general recognition that the grid mesh size also influences overall resolution. The publication accurately summarizes the state of the art in grid energy resolution as of 1972 which it is believed was also pertinent as of the time of this invention. Typically, energy resolution is in the order of one part in 200. One of the critical limiting aspects of achieving higher degree of resolution is the precision in which the fine mesh grids can be made spherical. Consequently, as set forth in the prior art, a variety of techniques have been proposed for the manufacture of such grids to maintain spherical configuration. As set forth herein, U.S. Pat. No. 3,864,797 and Japan Kokai No. 55-62790 are representative of various attempts within the art. Given manufacturing constraints, however, there is a size limitation inherent in all such prior art manufacturing processes. Consequently, a requirement for a larger device carries with it the co-commitment loss in energy resolution. Prior to this invention, it is believed that no system provided the necessary flexibility that the virtual elimination of size as a function of energy resolution which could be obtained in the spherical retarding grid analyzer.